Lead zirconate titanate containing lead is a typical piezoelectric material, and has been used in various piezoelectric devices, e.g., actuators, oscillators, sensors, and filters. However, a lead component in a discarded piezoelectric material may be eluted into the soil and may have a harmful influence on an ecosystem. Therefore, in order to produce lead-free piezoelectric devices, research and development of lead-free piezoelectric material have been performed intensively.
Typical lead-free piezoelectric materials which are widely researched at present are piezoelectric materials containing potassium niobate. However, when the piezoelectric material containing potassium is synthesized, the hygroscopicity of a raw material (for example, potassium carbonate) powder is high, so that it is difficult to accurately weigh the raw material powder at a predetermined molar ratio. A piezoelectric material containing potassium niobate (KNbO3) has deliquescence, and the piezoelectricity of piezoelectric ceramic containing potassium niobate may be degraded with time. In addition, there is a problem in that ceramic having a high relative density is not obtained easily from the piezoelectric material containing potassium niobate.
NPL 1 discloses a material represented by (1−x)(0.1BaTiO3-0.9NaNbO3)-xLiNbO3 (where 0≦x≦0.125). The Curie temperature is increased by allowing 0.1BaTiO3-0.9NaNbO3 to make a solid solution with LiNbO3. The Curie temperatures when x=0.01 and 0.02 are 526 K (253° C.) and 531 K (258° C.), respectively. However, there is a problem in that in the case where the amount of LiNbO3 is 1% or more, a high-density sample is not obtained. Also, there is a problem in that if the amount of LiNbO3 increases, both the magnitude of spontaneous polarization and the magnitude of piezoelectric constant d31 decrease.
NPL 2 discloses that when CuO is added to 0.9NaNbO3-0.1BaTiO3 by up to 0.3 percent by mole, the relative density of a sintered body increases, pores decrease, the piezoelectric constant (d33), the electromechanical coupling coefficient (kp), and the mechanical quality factor (Qm) increase, and the dielectric loss tangent (tan δ) decreases. However, there is a problem in that if the amount of addition of CuO is 0.4 percent by mole or more, the relative density of the sintered body, d33, kp, and Qm decrease because an impurity phase is generated.